Amplifiers, and, particularly, operational amplifiers and differential amplifiers, are well-known in the art. Such amplifiers have a variety of applications, such as described, for example, in Design of MOS VLSI Circuits for Telecommunications, edited by Y. Tsividis and P. Antognetti, published by Prentice-Hall, Inc. (1985), and herein incorporated by reference. Typically, it is desirable to ensure that such differential amplifiers and operational amplifiers operate linearly so that the output voltage signal is approximately an amplified version of the applied input voltage signal. However, it is sometimes also desirable to produce harmonic components of an applied voltage signal.